Provided by: filtergen_0.12.4-5.1ubuntu1_amd64 
NAME
rules.filter - Input format for filtergen packet filter compiler
INTRO
This file describes the input syntax accepted by filtergen(8).
BASICS
In general form, a filter rule is described by a direction, an interface, a target and
(possibly empty) sets of matches and options.
Simple rules will look like:
direction interface match0 .. matchN target;
for example:
input eth0 source host1 dest host2 proto tcp dport http accept;
Note that the elements of the rule can be placed in any order, with the exception that the
interface must immediately follow the direction. Thus, this rule is equivalent to the
above (though perhaps less readable):
proto tcp source host1 dport http accept dest host2 input eth0;
The semicolon separates rules. It is optional before a closing brace or the end of a
file. Whitespace is not significant. Anything after a hash ("#") on a line is ignored.
DIRECTION
A direction merely specifies whether to match packets being sent or received. The only
two directions available are "input" and "output". Forwarded packets will pass through
both,
INTERFACE
This specifies which real or virtual network device to filter. As far as filtergen is
concerned, this is just a text string. It must be the same as the device name on the
target system. Common names on Linux are "eth0", "eth1", ..., "ppp0", etc. Other systems
will have different naming rules.
TARGET
A target notes what we do with a matching packet. Universal options are accept and drop
which, respectively, state that the packet should be allowed as normal, or thrown away.
Some backends support reject to throw away a packet, but send notification to the sender
that it was denied, masq (on output rules only) to "masquerade" a packet - alter it so
that it appears to come from the address of the sending interface - and proxy (and its
deprecated alias redirect) to divert a connection via the local system.
MATCHES
The matches are the meat of the rule. They apply a set of tests to a packet and decide if
this rule will be used to process it. Available matches are:
source addr-range dest addr-range
proto {tcp|udp|icmp|...} sport port-range dport port-range icmptype icmp-type
Matches can be negated by prefixing them with a "!":
input eth0 ! dest 10.0.0.3 reject;
(note than not all backends can support this).
OPTIONS
Options affect the behaviour of the matcher or the target. Currently implemented are log,
which logs packets, local, which means only to check packets to or from this interface,
forward which means the opposite of local, and oneway which causes the backend to omit
rules which permit return packets.
The log option can optionally specify a message to log matching packets with, where the
backend supports it:
input eth0 source {
10.0.0.0/8 192.168.0.0/16
} log text "private addresses" drop;
Note that the oneway option make have no effect when used with the -l command-line flag on
backends which support it.
GROUPING
Because it can get tedious to type:
input eth0 source foo dest bar proto tcp dport http accept;
input eth0 source foo dest bar proto tcp dport https accept;
input eth0 source foo dest bar proto tcp dport nntp accept;
input eth0 source foo dest bar proto tcp sport 1:1023 dport ssh accept;
...
filter allows you to group rules with a set syntax:
input eth0 source foo dest bar proto tcp {
dport http;
dport https;
dport nntp;
sport 1:1023 dport ssh;
} accept;
Matches which accept arguments can also be grouped:
input eth0 source foo dest bar proto tcp {
dport {http https nntp};
sport 1:1023 dport ssh;
} accept;
OUT-OF-LINE GROUPS
It is commonly the case that both hosts and routers have long lists of similar looking
rules to allow traffic between groups of hosts, as above. What if we had another pair of
hosts which needed a variety of services? We could simply put the rule groups one after
the other:
input eth0 source foo dest bar proto tcp {
dport {http https nntp};
sport 1:1023 dport ssh;
} accept;
input eth0 source baz dest quux proto tcp {
dport {1264 1521 1984 8008 8080 26000};
} accept;
The above generates 11 rules, and every additional port adds another rule through which
packets will pass (well, ones which don't match any of the above). The first four output
rules have the same source and destination hosts and protocol, and we know that if it
doesn't match those on the first rule, it won't on the next three, either. Out-of-line
groups use this fact to streamline things somewhat:
input eth0 source foo dest bar [
proto tcp {
dport {http https nntp};
sport 1:1023 dport ssh;
} accept;
];
input eth0 source baz dest quux [
proto tcp { dport {1264 1521 1984 8008 8080 26000}; } accept;
];
Where the underlying system supports it, everything inside the square brackets is moved
into a separate "chain" (in ipchains and iptables-speak) or "group" (in ipfilter-speak).
Thus, any packet not matching "source foo dest bar" or "source baz dest quux" above will
be checked against only two rules, not eleven.
Note that matches which must appear together, like "proto tcp" and "sport 12345" need to
be either both in the group, or both out of it.
EXAMPLE
Here's a fairly complete example, for a single-interface machine:
#
# Example filter for (for example) a mail server
#
# Unfortunately, we don't have time to audit the
# communications which go on locally
{input lo; output lo} accept;
# But we want to be a bit more careful when speaking
# to the outside world
input eth0 {
# Sadly, we share a DMZ with Windows machines.
# Don't log their netbios noise
proto {tcp udp} source ournet/24 dport 137:139 drop;
proto tcp {
dport { smtp pop-3 } accept;
dport ssh source ournet/24 accept;
# We don't answer this, but don't want to
# cause timeouts by blocking it
dport auth reject;
log drop;
};
# We don't run any UDP (or other non-TCP)
# services
log drop;
};
output eth0 {
proto tcp {
dport { smtp auth } accept;
log drop;
};
# Outbound DNS is OK
proto udp dport domain dest { ns0 ns1 } accept;
log drop;
};
SEE ALSO
filtergen(8), filter_backends(7)
January 7, 2004 FILTER SYNTAX(5)